(301b) Effect of Cluster Size On CO Adsorption and Dissociation On Cobalt Catalysts: DFT Studies Using Cluster Models

CO adsorption and dissociation is the first and probably the most important step in Fischer-Tropsch (FT) reaction. In this article we explore the influence of cobalt particle size on CO adsorption and dissociation. In small particles the reactivity tends to increase due to the increase in the relative ratio of surface atoms and atoms which have lower coordination numbers¹. Smaller particles lead to increased coordinatively unsaturated sites at which the adsorption energies are the highest. However, decreasing the particle size to very small ensembles can lead to reduced activity. This is due to the very strong bonds of the low coordination number atoms with the adsorbate to compensate for the smaller number of metal-metal bonds.

Experimental studies showed that activity and selectivity of FT reaction were affected by cobalt particles size². Planar models are widely used to study the reaction mechanisms on flat and stepped cobalt surfaces³. In this work, Density Functional Theory (DFT) is used to study the effect of catalyst size on CO adsorption for Cobalt clusters ranging from 1 nm to 8 nm. Cobalt nanoparticles form ß-multiply-twined icosahedrons (MT) below 6 nm and ß-Wulff-polyhedrons (WP) in the size range from 6 to 110 nm⁴. MT particles expose twenty (111) faces and WP particles expose eight (111) and six (100) faces. Three-dimensional models are chosen to represent these clusters exposing the predominant faces as well as edge sites because the conventional planar cluster models do not consider edge atoms which can be one of the most preferred sites for CO adsorption. The most favorable site for CO adsorption on these clusters is examined and the effect of cluster size on CO adsorption is presented.

References

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